The present invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for forming a field insulator film surrounding an element region.
The element separating technique utilizing the selective oxidation of a silicon nitride film is widely adopted as a method for fabricating a MOSFET and an integrated circuit incorporating such MOSFETs.
The case for fabricating an integrated circuit incorporating n-channel MOSFET transistors by this method will be described with reference to FIG. 1.
As shown in FIG. 1A, after forming a silicon oxide film 2 on the surface of a p-type silicon substrate 1 by thermal oxidation, a silicon nitride film 3 is deposited thereover as an oxidation preventive film. After forming a resist film 4, the parts of the silicon nitride film 3 excluding the element regions to form a source region, a drain region and a gate electrode are selectively etched away by photolithography. While keeping the deposited resist film 4, boron is ion-implanted to form a field inversion preventive layer 5.
After removing the resist film 4, field oxidation is performed to form a field insulator film 6 to achieve the structure as shown in FIG. 1B. An oxide film 7 on the silicon nitride film 3 formed by this oxidation process, the silicon nitride film 3, and the underlying silicon oxide film 2 are sequentially etched away. The elements are formed according to the known method to complete fabrication of an n-channel MOS-type field effect transistor as shown in FIG. 1C. Referring to FIG. 1C, reference numerals 10 and 11 denote a source region and a drain region, respectively; 13, a gate electrode of polycrystalline silicon formed on a gate silicon oxide film 12 on a channel region between the source region 10 and the drain region 11; 14, an interlaid insulator film; and 15, an aluminum wiring layer.
The plan view of this transistor element is shown by the model view of FIG. 1D. A section of this plan view along the line a--a is shown in FIG. 1C, and a section along the line b--b is shown in FIG. 1E.
The conventional element separation by selective oxidation as described above has the drawbacks to be described below.
First, as shown in FIG. 1B, the edges of the silicon nitride film 3 are turned up, and the element region is reduced by .alpha. at each side in the longitudinal direction of the channel, and is also reduced by .alpha. at each side in the direction of the channel width as shown in FIG. 1E. For this reason, for obtaining one element region of a desired size as the final shape, the width and length of the resist film 4 must first be greater by 2.alpha. in both directions than the actual size required.
When a blank region is to be 3 .mu.m between element regions, between wirings utilizing impurity regions formed by diffusion of an impurity into a substrate, or between these two types of regions, the blank region between the edges of the resist film 4 of FIG. 1A becomes 1.5 .mu.m if conversion difference .alpha. by the selective oxidation is 0.75 .mu.m at each side. This results in technical difficulties. Although the conversion difference .alpha. may be reduced by thickening the silicon nitride film 3, this results in extra stress exerted on the edges of the element region and adversely affects the elements.
Secondly, boron in the field inversion preventive layer 5 formed by implantation of boron ions using the silicon nitride film 3 as a mask diffuses into the element region during field oxidation, and the channel width is thereby reduced by .alpha. at each side as shown in FIG. 1E. With an element having a small finished channel width of 3 .mu.m, for example, the reduction by .alpha. is significant. For this reason, the silicon nitride film 3 must be formed in a greater size considering this reduction.
Thirdly, the turning up of the edges of the silicon nitride film 3 under thermal oxidation exerts stress on the element region. This causes crystal defects and reduces the reliability of the transistor.